In the prior art, the emitting surface of a dispenser cathode is made from either porous metal matrices whose pores are filled with electron emitting material or porous metal plugs or perforated foils covering reservoirs of electron emitting material. The porous metal matrices and porous metal plugs exhibit a random porosity without consistently uniform pore size, pore length, or spacing between the pores on the surface. The electron emission is related to the surface work function reducing material trapped in the pores, which are of variable size and spacing. Accordingly, dispenser cathodes of the prior art do not have uniform surface electron emission.
FIG. 1 shows a prior art powdered tungsten sintered cathode 10. Tungsten powder grains 12 are sorted to a range on the order of 10 u and are compressed and sintered under elevated temperature to form a cathode 10 comprising a porous tungsten matrix. The matrix structure is then impregnated with a surface work function reduction material 30, such as BaO, CaO, and Al2O3. When operated as an electron source in a microwave gun, the cathode is heated to a temperature of approximately 1000° C. and a voltage 18 is applied between the cathode 16 and anode 17, which is shown as a conductive plate for simplicity. The impregnate work function reducing material (not shown) migrates through the pores 14 to the emission surface 16 and lowers the work function for electron emission, thereby improving the yield of free electrons 15. The voltage 18 is applied with sufficient potential for free electrons in the tungsten to overcome the surface work function voltage and be accelerated from the surface 16 to the anode 17. Ideally, the electron emission from cathode 16 should be uniform, however this is limited by the uniformity of deposition of work function reducing material through the cathode, which typically has irregular porosity, as was earlier described.
Others have proposed processes for manufacturing controlled porosity cathodes. In U.S. Pat. No. 4,379,979, Thomas and Green describe a technique using silicon and metal deposition. This process starts with a generally flat silicon template substrate structure having and array of upstanding microposts 1-25 microns across on 5-10 micron spacings from each other. A layer of metal is then deposited on the substrate to surround the microposts and cover the substrate to a desired depth. The metal layer is abraded to a smooth, flat surface which exposes the microposts. Thereafter, the silicon substrate and microposts are completely etched away, leaving a metal sheet having micron-size holes throughout. This technique is applicable to small, flat cathodes. It contains a number of process steps which limit both the size and configurations that can be obtained. The thickness of the cathode material is approximately 100 microns. This technique would not be applicable to large cathodes where differential thermal expansion could cause the material to buckle or warp.
In U.S. Pat. No. 4,587,455, Falce and Breeze describe a process for creating a controlled porosity dispenser cathode using laser drilling. In this process, a configured mandrel is coated with a layer of material such as tungsten so that when the mandrel is removed from the coating material a hollow housing is formed having a side wall and an end wall which define a reservoir. Thereafter an array of apertures is formed in the end wall of the housing by laser drilling to create an emitter-dispenser, but this method is only applicable to small cathodes, as the laser drilling process becomes unmanageable for large cathodes where millions of holes would be required. Also, the thin coating which forms the emitter is subject to warping and buckling from differential expansion of the coating and the support structure.
In U.S. Pat. No. 4,745,326, Green and Thomas describe a controlled porosity dispenser cathode using chemical vapor deposition and laser drilling, ion milling, or electron discharge machining for consistent and economical manufacture. This process is also more applicable for small cathodes where the number of laser drilled holes are manageable. This process also includes a large number of separate sequential processes to obtain the final cathode and can not provide cathode emitting surfaces of arbitrary thickness.
In U.S. Pat. No. 5,118,317, Wijen describes a process that uses an array of porous, sintered structures where the powder particles are coated with a thin layer of ductile material. Since this process begins with particles containing a distribution of sizes, there is no direct control of the porosity through the entire structure.
U.S. Patent Application 2002/0041140 by Rho, Cho, and Yang describes a process for oxide cathodes that controls the porosity and electron emission. This process is only applicable to oxide cathodes which are fundamentally different from the dispenser type of the present invention.
One application for the sintered wire process is the fabrication of X-ray anodes, which are typically formed from high atomic number metals such as tungsten or molybdenum, and form x-rays as secondary particles resulting from the collision of high energy electrons into a target surface. The electrons are accelerated from an electron gun at a large negative potential with respect to an anode, and the target anode is often at an angle to the incoming electron trajectory. This target angle encourages the secondary particles and x-rays to exit the x-ray target and pass through an aperture in the housing surrounding the X-ray tube, thereby forming an x-ray source.
FIG. 6a shows a prior art fixed anode X-ray tube 64, which comprises a heated cathode 66, an evacuated chamber (not shown), and a high thermal conductivity substrate 68, which includes a surface 65 which is formed from a material having a high melting temperature such as tungsten, molybdenum, tantalum, niobium, or any material with a high atomic number and associated high melting temperature compared to the high thermal conductivity substrate 68. In the prior art of x-ray tubes, the size of the x-ray target and density of the electron beam 67 is limited by the thermal conductivity of the target material and the heat load delivered to the x-ray 69 producing surface material 65.
FIG. 6b shows a rotating target prior art x-ray tube 70, where the heated cathode 78 generates an electron stream 79 which may be focused on a rotating surface 74, where the rotation is governed by a motor 72 which may be outside of the evacuated envelope (not shown). The substrate 76 may be comprised of a thermally conductive material such as copper, silver, gold, or graphite, which has applied on its surface a thin layer of x-ray 80 producing material 74 which may be tungsten, or molybdenum or any material or alloy suitable for the production of x-rays.
In the prior art, there is no control of the size and distribution of the pores 14 over the cathode surface 16. This results in non-uniform distribution of the work function reducing impregnate over the surface 16. In a dispenser cathode, a longer cathode lifetime is accomplished by maintaining a reservoir of work function reducing material behind a porous cathode having an emission surface, where the uniform porosity of the cathode expresses the work function reducing material to the emitting surface, resulting in a cathode with long emission times. Until the present invention, it has not been possible to fabricate a uniformly porous cathode of variable diameter or thickness for this purpose.
It is desired to provide a uniform porosity tungsten cathode which may be used as a dispenser cathode having an emission surface and a dispenser surface adjacent to a source of work function reducing material. It is also desired to provide a method for the fabrication of a uniform porosity cathode. It is also desired to provide a porous cathode structure having uniform porosity where such porosity is invariant through the structure, such that many cathodes of arbitrary thickness may be formed from the structure.
FIG. 2a shows two generalized sintering progression curves for sintered copper wires at the copper sintering temperatures 1000° C. and 1050° C., where the progression of sintering is measured by the closing of pores over time as described in “Fundamental Principles of Powder Metallurgy” by W. D. Jones, Edward Arnold Publishers, London, 1960. The sintering progression is expressed in the metric(r03−r3)/a3, where
r0 is the initial effective radius of the pore
r is the effective radius of the pore at time t
a is the initial radius of the wire.
The progression of time and temperature reduces the pore size as shown in FIGS. 2b through 2d. FIG. 2b shows the initial condition for time t=0 where the sintered structure 20 comprises a plurality of copper wires 22, with initial pores 24 formed by the spaces between the wires 22. After application of a sintering temperature T such as 1000° C. for copper wires for a time t=T1, the pores 24 begin to close as the wires 22 sinter together, as shown in FIG. 2c. At a final time t=T2 shown in FIG. 2d, the pores 24 have further closed as the wires sinter together to form a continuous porous structure. By careful selection of sintering time and pressure, the desired porosity may be achieved in the cathode structure 20.
Sintering of copper wires in the prior art has been used principally to develop sintering models and to understand the sintering process for particles, which are treated in the limit as spheres, and has not been used to form continuously porous structures, such as would be used for dispenser cathodes for electron emission.
Devices using electron beams may generate these beams using dispenser cathodes. These porous cathodes are impregnated with material designed to lower the work function at the cathode surface. The cathode is heated to approximately 1000° C. and the impregnate migrates through the pores in the tungsten to the surface. Problems occur when the distribution of pores varies across the cathode surface, leading to nonuniform migration of the impregnate. When this occurs, there is a variation in emission of electrons caused by the variation in work function. This is particularly troublesome for cathodes operating in a regime where the emission is dependent on the temperature. In these circumstances, the emission variation can vary greatly over the surface.
In addition to the fabrication of cathodes for use in electron tubes, other additional applications for sintered wire rods may be envisioned. One such application is the use of targets to generate secondary particles such as X-rays from high energy collisions, where the target for the high energy electrons or other particles naturally accumulates large amounts of thermal energy from such collisions, compared to the energy of the released x-rays, and the heat must be removed to prevent melting of the target. In one such application, x-ray targets are formed from high melting point metals such as tungsten or molybdenum, which form the anode of an x-ray generating device. Presently, the start of the art for x-ray tube anode thermal control involves concentrating the incoming electron beam on a small part of the tungsten anode, and rotating a large area of target anode through the electron impingement region, such that the active target area is heating while other parts of the rotating anode are drawing thermal energy from the region of impingement.
Rotating anode x-ray sources are described in U.S. Pat. Nos. 4,165,472 by Wittry, 4,920,551 by Takahashi et al, 4,958,364 by Guerin et al, 4,991,194 by Laurent et al, 6,560,315 by Price et al, and 6,735,281 by Ohnishi et al. U.S. Pat. No. 6,430,264 by Lee describes the use of carbon fibers in a rotating anode for improved thermal conductivity from a tungsten target to the underlying substrate.
U.S. Pat. No. 5,943,389 by Lee describes an x-ray target comprising a substrate which is coated with perpendicularly oriented high thermal conductivity fibers, whereafter a layer of high atomic number x-ray producing material is applied.